FIG. 15 is a view for explaining the principle of the ultrasonic flow amount measuring device. In an ultrasonic flow amount measuring device 200 shown in FIG. 15, fluid to be measured flows in a direction indicated by an arrow FD in a conduit pipe 60. A pair of ultrasonic transducers 70 is disposed on an outer surface of the conduit pipe 60 with a predetermined distance therebetween. Each of the ultrasonic transducers 70 includes a vibrator 71 and a plastic wedge 72. An acoustically coupling medium 79 for acoustic matching is coated between the wedges 72 and the outer surface of the conduit pipe 60 to enhance propagation efficiency of the ultrasonic waves. An ultrasonic wave transmitted from one of the ultrasonic transducers is reflected at an inner surface of the conduit pipe 60, and received by the other of the ultrasonic transducers as indicated by a line with arrows shown in FIG. 15. A plane containing a propagation path of the ultrasonic wave is referred to as a propagation plane. The propagation plane corresponds to a sheet face of FIG. 15 containing an axis line of the conduit pipe 60.
When the ultrasonic transducers 70 are properly oriented and positioned so that the ultrasonic wave travels on the propagation plane, a time difference Δt in arrival times at the ultrasonic transducers (propagation time difference) of the ultrasonic wave traveling in a forward direction and a backward direction relative to the flowing direction is expressed as follows:Δt=2LV/c2  (1)
where V is a velocity of the fluid flowing in the conduit pipe 60, L is a length of the propagation path, and c is the speed of sound. From Equation (1), when the propagation time difference Δt is known from a measurement, the velocity of the fluid is determined. Also, when a cross-sectional area of the conduit pipe 60 is known, a flow rate of the fluid flowing through the conduit pipe is obtained.
FIG. 16 is a plan view showing an example of a conventional ultrasonic flow amount measuring device constructed on the basis of the above-described principle. FIG. 17 is an enlarged plan view showing a transducer shown in FIG. 16, and FIG. 18 is an enlarged front view showing a further detail of the transducer shown in FIG. 16. In the ultrasonic flow amount measuring device 201 shown in FIG. 16, a guide rail 80 is provided for linearly guiding a pair of ultrasonic transducers 70 parallel to an axial line of the conduit pipe 60. The guide rail 80 has two frames and is disposed parallel to the axial line of the conduit pipe 60. The guide rail 80 sandwiches the ultrasonic transducers 70 on side surfaces thereof from both sides. Both ends of the guide rail 80 are joined to saddles 81. The saddles 81 are secured to the outer peripheral surface of the conduit pipe 60 by belts 82 formed of metal or plastic.
The ultrasonic transducers 70 are linearly guided along the guide rail 80. The ultrasonic transducers 70 are positioned so that transmission/reception of the ultrasonic wave is carried out along the line RT with arrows shown in FIG. 15. As shown in FIG. 18, a positioning shaft 78 is vertically provided at a bottom surface of the seat 72 of each transducer 70. A screw member (not shown) having a cylindrical shape is provided to be coaxial with the positioning shaft 78, and has screw threads formed on inner and outer sides thereof. The screw thread on the outer side of the screw member engages a screw thread formed on an inner side of a position fixing screw 75. The screw thread on the inner side of the screw member engages a screw thread formed on an outer side of a height adjusting screw 76. When the position fixing screw 75 is rotated, the guide rail 80 is sandwiched by an upper surface of a plate member (not shown) connected to the bottom portion of the screw member and the lower surface of the position fixing screw 75. Accordingly, the ultrasonic transducers 70 are fixed so as not to move in the axial direction of the conduit pipe 60. When the height adjusting screw 76 is rotated, the transducer 70 moves upwardly and downwardly with respect to the screw member. Accordingly, in the fixing step, a distance between the ultrasonic transducers 70 is adjusted so that a cursor 85 attached to each of the ultrasonic transducers 70 is positioned at a specific position on a scale attached to the guide rail 80. Then, each of the ultrasonic transducers 70 is fixed at the position by the position fixing screw 75, and is closely attached to the upper surface of the conduit pipe 60 by the height adjusting screw 76.
In the above-described conventional device, it is necessary to accurately position the ultrasonic transducers with an appropriate distance therebetween to place the axial line of the conduit pipe on the propagation plane of the ultrasonic wave, so that the ultrasonic transducers precisely perform the transmission and reception of the ultrasonic wave along the line RT. In the conventional device, the distance between the ultrasonic transducers is adjusted by positioning the cursors attached to the ultrasonic transducers based on the scale attached to the guide rail. The ultrasonic transducers are fixed at the proper positions of the guide rail, and are closely attached to the upper surface of the guide pipe with the fixing screws and the height adjusting screws. Therefore, it is difficult to fix the ultrasonic transducers with high precision, thereby taking long time and requiring a skilled worker. Further, the acoustic coupling medium is coated on the ultrasonic transducers, and the guide rail is fixed on the outer surface of the conduit pipe with the belts while positioning. Accordingly, the fixing screws may be loosened to cause a displacement in the distance between the ultrasonic transducers. The acoustic coupling medium may be exfoliated or hardened, and the ultrasonic transducers may not closely contact the conduit pipe, thereby causing an adverse effect.
In view of the above problems, the present invention has been made, and an object of the present invention is to provide an ultrasonic flow amount measuring device in which each of the ultrasonic transducers can be accurately positioned with a right angle at a right position without difficulty, thereby improving workability and reducing a working time.